The wandering rat: response to Suddendorf

Abstract

It was Thomas Suddendorf [1xMental time travel: continuities and discontinuities. Suddendorf, T. Trends Cogn. Sci. 2013; 17: 151–152Abstract | Full Text | Full Text PDF | PubMed | Scopus (21)See all References[1] who sparked my interest in mental time travel and, at his instigation, we set up what we thought were the necessary criteria to establish behavioral evidence for mental time travel in nonhuman species. These criteria are perhaps yet to be convincingly met in animal research. They are, however, essentially behavioral. Neurophysiology can provide a further avenue of investigation and arguably give a better perspective on what is ‘mental’ about mental time travel.Hippocampal activity is clearly associated with mental time travel in humans, and parallels are increasingly evident from hippocampal recording in the rat. I have already noted evidence that activity in the rat hippocampus when the animal is outside of a maze signals previous paths within the maze, as well as paths not traversed, raising the possibility of anticipated future activity [2xHippocampal replay is not a simple function of experience. Gupta, A.S. et al. Neuron. 2010; 65: 695–705Abstract | Full Text | Full Text PDF | PubMed | Scopus (159)See all References[2]. Further evidence of activity signaling future behavior comes from rats trained to alternate left and right turns at a particular location in a maze. Between trials, the rats were introduced to a running wheel and, while they were running, differential activity in the hippocampus signaled which turn they would take next. (My mind wanders, too, when I’m on a treadmill). Based on this and other findings, the authors concluded that self-organized activity in the hippocampus, ‘having evolved for the computation of distances, can also support the episodic recall of events and the planning of action sequences and goals’ ([3xInternally generated cell assembly sequences in the rat hippocampus. Pastalkova, E. et al. Science. 2008; 321: 1322–1327Crossref | PubMed | Scopus (334)See all References[3] p. 1327).Mental time travel in the rat, if indeed it exists, is no doubt less complex than it is in humans. As Suddendorf suggests, the critical question is whether this amounts to a fundamental discontinuity. At one extreme, one might argue that the evidence based on hippocampal recording in the rat is already evidence for mental time travel. At the other extreme, one might consider the added complexity sufficient to make human mental time travel different in kind. To be sure, there is no evidence that the imagined wandering of the rat includes other individuals, things, and complex actions in addition to sequences of locations, as it does in humans. Neither is there evidence for the sheer number of real and imagined events that we humans carry or create. Nevertheless, it would not be surprising if mental time travel evolved from brain networks, present in rats as in humans, adapted for the representation of space and of sequences of locations within space, and even for keeping track of time [4xAssembly sequences arising from spike threshold adaptation keep track of time in the hippocampus. Itskov, V. et al. J. Neurosci. 2011; 31: 2828–2834Crossref | PubMed | Scopus (42)See all References[4]. Moreover, our common ancestry with the rat goes back some 75 million years, so evidence from the rat should, if anything, understate the case for interspecies continuity.On the question of human uniqueness, it may also be pertinent to ask where the burden of proof lies. We have a strong tradition of respecting the principle of parsimony, whereby the simplest of possible explanations is to be preferred. Explanations are considered ‘simple’ to the extent to which they do not involve higher thought processes, such as language or, for that matter, mental time travel. The ‘Clever Hans’ debacle is often cited as an example of failure to apply this principle [5xSee all References[5]. By contrast, applying overly simple explanations for animal behavior can lead to smug superiority and the invocation of miracles, such as the ‘great leap forward’ to explain why only humans are capable of language [6xBiolinguistic explorations: design, development, evolution. Chomsky, N. Int. J. Philos. Stud. 2007; 15: 1–21Crossref | Scopus (27)See all References[6]. From a Darwinian perspective, it might sometimes be more prudent to assume differences in degree rather than in kind as the default position, and seek evidence that this is not the case.